An ink-jet printing apparatus that forms an image on a printing medium by ejecting ink droplets onto the medium from a printhead has been proposed as an apparatus for printing documents and images generated by a computer. An ink-jet printer generally expresses an output image by absence or presence of dots formed by the ink droplets.
Accordingly, a printer driver installed in a host device connected to such an ink-jet printer sends the ink-jet printer print data, which is obtained by subjecting an input image acquired from an application program to color conversion processing and then executing binarization processing by pseudo-gray-scaling means such as the error diffusion method, and the inkjet printer forms an output image by ejecting ink droplets in accordance with the print data received.
The error diffusion method is described in “An Adaptive Algorithm for Spatial Gray Scale”, by R. Floyd, et al., SID International Symposium Digest of Technical Papers, vol. 4.3, 1975. As shown in FIG. 4, this method, where P represents a pixel of interest, v the density of this pixel, v0, v1, v2 and v3 the densities of unbinarized pixels P0, P1, P2 and P3, respectively, neighboring the pixel of interest P, and T a threshold value for binarizing these densities, is a method of diffusing binarization error E at the pixel of interest P to the neighboring pixels P0, P1, P2, P3 by weighting coefficients W0, W1, W2, W3, respectively, determined empirically, thereby making mean density of the output image equal to the density of the input image macroscopically.
Errors E0, E1, E2, E3 with regard to the neighboring pixels P0, PI, P2, P3 can be obtained by the following equations, where o represents output binary data:if v≧T holds, then o=1, E=v−Vmax; if v≦T holds, then o=0, E=v−Vmin;                 where Vmax and Vmin represent maximum and minimum densities, respectively; andE0=E×W0  (1)E1=E×W1  (2)E2=E×W2  (3)E3 E=W×3  (4)        
Known examples of weighting coefficients are W0= 7/16, W1= 1/16, W2= 5/16, W3= 3/16.
Since error diffusion processing is executed one raster at a time in the main-scanning direction, generally error in the sub-scanning direction (paper transfer direction) is stored in a storage device referred to as an error buffer and is used at the processing of the next raster.
In a case where a plurality of pages are printed, the image quality of the leading end (beginning) of a page deteriorates when error from the trailing end of the preceding page propagates to the leading end of the aforesaid page. The general practice, therefore, is to initialize the error buffer when a new page begins. Similarly, in a case where a plurality of images 602 and 603 are arrayed in the sub-scanning direction on the same page 601, as shown in FIG. 6, image quality at the upper end of the image 603 deteriorates if error in the sub-scanning direction that has occurred at the lower end of the image 602 propagates to processing of image 603. In a known method, therefore, a deterioration in the image quality at the upper end of the image 603 is similarly prevented by initializing the error buffer immediately before binarization processing of the image 603 is started.
A printing apparatus of the above kind is capable of printing not only on so-called single-cut sheets (referred to as “cut sheets” below) but also on elongated sheets (referred to as “banner sheets” below) such as roll paper or banner paper (such printing shall be referred to as “long-length printing”). In this case, the amount of data that can be handled by an application program generally is limited. When long-length printing is performed, therefore, in general a printer driver is supplied with image data on a per-page basis in a form in which the data, which is for printing one image 702 on an elongated printing medium 701 of the kind shown in FIG. 7, is divided into a plurality of pages.
However, if an image that is originally a single image is simply divided into a plurality of pages and then binarized, a deterioration in image quality occurs at the page boundaries because the error buffer is initialized at these boundaries.
Methods of solving this problem are disclosed in the specifications of Japanese Patent Application Laid-Open Nos. 2000-158714 and 10-329383. These methods prevent a deterioration in image quality at page boundaries by not initializing the error buffer at the boundary of each page when long-length printing is carried out.
With the methods described in the above-mentioned specifications, however, initialization of the error buffer is not always performed at the time of long-length printing. As a consequence, binarization error is propagated between different images and a deterioration in image quality results.
For example, when long-length printing of a banner sheet 801 having a length of two cut sheets and including images 802, 803 is performed, binarization error at the lower edge of the image 802 propagates to the image 803, regardless of the fact that the two images 802 and 803 are separate objects, and therefore a deterioration in image quality occurs.